Heitler-London molecular orbital theory

This can be given a simple quantitative description by means of the Heitler-London molecular orbital theory, but this requires more details than is appropriate in this text. Appendix I provides some of the details for interested readers. Only an outline will be given here. 

Historically, molecular orbital theory was preceded by an alternative and successful description of the bonding in H2. In 1927, W. Heitler and F. London proposed the valence bond theory, in which each electron resides in an atomic orbital. In other words, in this model, the identity of the atomic orbital is preserved. There are two ways in which the two electrons in H2 can be accommodated in the pair of Is atomic orbitals ... 

Figure 3.4. (a) The overlap of two one-electron atomic wave functions, each centered on a different atom, constitutes the Heitler-London (valence-bond) theory, (b) A one-electron molecular wave function, or molecular orbital, in the molecular orbital theory of Hund and Mulliken. 

Molecular orbital theory originated from the theoretical work of German physicist Friederich Hund (1896-1997) and its apphcation to the interpretation of the spectra of diatomic molecules by American physical chemist Robert S. MuUiken (1896-1986) (Hund, 1926, 1927a, b Mulliken, 1926, 1928a, b, 1932). Inspired by the success of Heitler and London s approach, Finklestein and Horowitz introduced the linear combination of atomic orbitals (LCAO) method for approximating the MOs (Finkelstein and Horowitz, 1928). The British physicist John Edward Lennard-Jones (1894-1954) later suggested that only valence electrons need be treated as delocalized inner electrons could be considered as remaining in atomic orbitals (Lennard-Jones, 1929). 

Hamiltonian, 139 Heitler-London model, 141-142 ion and molecular orbital theory, 157-160... 

In 1927, Burrau calculated the energy of Hj and Heitler and London treated the hydrogen molecule. In 1928, the Heitler-London or valence bond method was applied to many electron systems, and simultaneously Hund and Mulliken started the development of the molecular orbital theory. In 1931, Slater expressed the v/avefunctions of complex molecules in terms of Slater determinants made up of linear combinations of atomic orbitals. Thus, the Golden Age was born. 

The electronic structure of molecules and atoms has long been at the focus of chemists interests. Soon after the electron was discovered in 1897, electron theories of valence were developed [1, 2]. However, at that time, nothing was known about the driving force behind the formation of a chemical bond or an ion. The discovery of quantum mechanics in 1925 made it possible to address the fundamental questions of chemistry. Schro-dinger s equation [3] became the central point in electronic structure theory. In 1927, Condon [4] gave a quantum mechanical explanation of the bond in H2 and initiated molecular orbital theory. In the same year, Heitler and London [5] developed the valence-bond description of H2. 

The first quantum-mechanical treatment of the hydrogen molecule was by Heitler and London in 1927. Their ideas have been extended to give a general theory of chemical bonding, known as the valence-bond (VB) theory. The valence-bond method is more closely related to the chemist s idea of molecules as consisting of atoms held together by localized bonds than is the molecular-orbital method. The VB method views molecules as composed of atomic cores (nuclei plus inner-shell electrons) and bonding valence electrons. For H2, both electrons are valence electrons. 

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